The relation of cogredience may fail in either of two ways.

One reason for failure may be that the part does not extend throughout the duration.

In this case the part may be cogredient with another duration which is part of the given duration, though it is not cogredient with the given duration itself.

Such a part would be cogredient if its existence were sufficiently prolonged in that time-system.

The other reason for failure arises from the four-dimensional extension of events so that there is no determinate route of transition of events in linear series.